1) Field of the Invention
The present invention relates to an apparatus for supplying a current to a laser diode, to cause the laser diode to emit a light to be used when reading digital data from an optical storage medium and when writing digital data thereto.
2) Description of the Related Art
Recently, many optical storage devices using an optical storage medium such as an optical disk, an optomagnetic disk, or an optical memory card have been developed. In such optical storage devices, a laser diode is used as a light source when carrying out a read operation and a write (or erase) operation. For example, during a write mode, a laser beam is made or projected incident to an area of a magnetic film of an optical storage medium which has been premagnetized, and as a result, that area of the magnetic film is heated. Accordingly, the direction of magnetization therein is inverted as compared to the of magnetization of adjacent areas thereof. Also, during an erase mode, a laser beam is projected incident to an area of the magnetic film to heat the area, and the heated area is then magnetized by using an external bias magnetic field. During a write or erase mode, a laser beam is used as the source of heat, and therefore, the driving of a laser diode for generating such heat requires a large amount of energy. On the other hand, during a read mode, when a laser beam is incident to an area of the magnetic film, the read data is determined by a change in a plane of polarization of a reflected light, due to the Kerr effect. Thus, in constrast, during a read mode, a laser diode for generating a laser beam is driven by a small amount of energy.
When the laser diode is driven by a small amount of energy, the output level of the laser diode is in a noise region, and the signal-to-noise (S/W) ratio of a reproduced signal is low. Therefore, to enhance this S/N ratio, a current supplied to the laser diode is modulated by a high frequency signal having a frequency much higher than a frequency used for recording data in the magnetic film, to enhance the peak of current supplied to the laser diode without increasing an effective output of light therefrom.
Also, to suppress fluctuations in the output of light emitted from the laser diode, an automatic power control (APC) technique is adopted, to bring a mean value of a light output during a read mode and a minimum light output (called a base read output) during a write mode close to a desired level.
In the above-mentioned laser diode drive system using an APC, although the desired level of the APC is not changed even when the control is transferred from a read mode to a write (or erase) mode, or vice versa, the drive current of the APC is changed, since the drive current of the laser diode, i.e., the illumination thereof during a read mode, is alternating current (AC), and the current provided during a write mode for a base read output is direct current (DC). Therefore, upon a switching between a read mode and a write (or erase) mode, the control of the light output is unstable due to a delay in the response caused by a time-constant of the APC, which reduces the speed of accessing to the optical storage medium, as later explained in detail.